This invention relates to an electrical circuit arrangement for preselecting the speed of an electric motor of a centrifuge. The circuit has an upstream inverter with a first series circuit, which consists of two controllable electronic switches constructed as field-effect transistors, connected to a direct-current source. A connection branches off between the field-effect transistors to a first terminal of the electric motor, the second terminal of the motor being connected to a second series circuit, which is also connected to a direct current source, of at least two electrical components as the tie point of these components, the triggering of the electronic switches being performed by a control logic.
In centrifuges of the type shown in brochure 2C2.85/VN Ku of Heraeus-Christ GmbH, the drive consists of an asynchronous motor, upstream of which is connected a three-phase inverter with three semiconductor bridges. The principle of construction of such an inverter is known, for example, from "Leistungselektronik" (Power Electronics) by Rudolf Lappe, Springer-Verlag, 1988, page 267 and pages 276-280. The triggering of the motor is performed from a control logic with a pulse generator, frequency divider and programmable counter, each of the six field-effect transistors being triggered via a driver and its own transformer coupling. The speed adjustment achievable with the aid of frequency dividers is set by means of a speed preselection control and checked with the aid of a display instrument. After termination of the centrifuging process, the asynchronous motor is operated as a generator, and the braking energy generated is dissipated in a separate braking resistor cooled by a fan.
The construction of the device is relatively complicated, the six electronically controlled switches requiring an equally large number of triggering elements, i.e. transformers, control signal amplifiers and a corresponding number of outputs of the control logic. The use of a microprocessor to undertake the tasks of the control logic requires a considerable programming outlay. The triggering electronics becomes very complicated, particularly when different speed ranges are to be set according to a prescribed program. A further problem is in the relatively complicated braking process, since in addition to dedicated reversing electronics, additional heat dissipation measures are required.
It is the object of the present invention therefore to provide a reduction in the number of controllable switches as well as in the control outlay and the power losses. It is a further object to provide braking of the centrifuge's drive motor without additional components such as a braking resistor or fan.